- Enable SSH access on original firmware's web interface
- Login via SSH (I used PuTTY on Windows), use the same login credentials as for the web interface
- Make a copy of ubi0_1: cp /dev/ubi0_1 /tmp
- Make a copy of ubi0_2: cp /dev/ubi0_2 /tmp
- Copy those two files (/tmp/ubi0_1 and /tmp/ubi0_1) to your PC via SCP (I used WinSCP on Windows)
- The two files are/must be identical in content, the first is your calibration data, the second is a backup copy
- Go to Zyxmon's AC58U builds page (https://lede-ac58u.zyxmon.org/firmware/) and download the XXX-squashfs-flash-factory.trx file.
- Get the latest OpenWrt release (XXX-squashfs-sysupgrade.bin)
- On the router's web interface, go to Administration -> Firmware Upgrade
- Upload the .trx file (Zyxmon) and wait until the router has rebooted into OpenWrt
- Delete jffs2 partition by using SSH on 192.168.1.1, login as root/no password and enter: umount /mnt/ubi0_5 (in case the partition is mounted) and then: ubirmvol /dev/ubi0 -N jffs2
- Go back to the browser on 192.168.1.1, login (without a password) and proceed to System -> Flash Firmware
- Flash the latest OpenWrt release (XXX-squashfs-sysupgrade.bin). Make sure to uncheck 'Keep settings.'
- You now have the latest OpenWrt on your router.
From what I understand, it is best to always do a software reboot (System -> Reboot) after changing any settings before pulling the power cord. (A power loss without reboot might corrupt data)